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Built-in project · Raspberry Pi Pico W

Pico W micro switch on GP14

MicroPython on a Pico W: a lever micro switch with its common on GP14 reads 1 while pressed and 0 at rest, and prints each change. The whole build, a Raspberry Pi Pico W and 3 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

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Beginner About 15 minutes to build and run. Runs in your browser. Free, and no account needed.

The parts retail for about $6.55. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

  • Common, normally closed and normally open contacts
  • Reading a switch that drives the pin both ways
  • Smoothing noise with a capacitor
  • Reacting only to a change in state
  • Difficulty: Beginner
  • Time: About 15 minutes
  • Board: Raspberry Pi Pico W
  • Parts: 5

How it works

The lever microswitch has a common leg on GP14. At rest, the common leg touches the NC leg, which goes to ground, so GP14 reads 0. When the lever is pressed, the common leg touches the NO leg, which goes to 3.3 V, and GP14 reads 1.

A 10k resistor to ground and a 100 nF capacitor on the same pin keep the signal steady. The program checks every 20 ms and prints a message only when the state changes.

Parts list

5 parts, plus the jumper wires. They retail for about $6.55; on Mokxi they're free.

Kits that can build it

This starter kit has every part in the circuit. Each kit's page lists what else it can build.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pinConnects to
Limit switch, lever microswitch pin COMRaspberry Pi Pico W pin GP14
Resistor, 10k Ω pin 1Raspberry Pi Pico W pin GP14
Capacitor, 100 nF pin 1Raspberry Pi Pico W pin GP14
Limit switch, lever microswitch pin NCGround
Limit switch, lever microswitch pin NO3.3 V
Resistor, 10k Ω pin 2Ground
Capacitor, 100 nF pin 2Ground

The code

The MicroPython code exactly as the editor opens it. Change a line there and press Run: the board runs it on the official MicroPython firmware, with nothing to compile.

main.py · Raspberry Pi Pico W
# Press gently: the micro switch's common leg is on GP14. Resting, it
# touches the ground leg; pressed, the 3.3 V one, so GP14 reads 1 while
# the lever is down.
from machine import Pin
import time

lever = Pin(14, Pin.IN)
was = 0

while True:
    now = lever.value()
    if now != was:
        print("lever pressed" if now else "lever released")
        was = now
    time.sleep_ms(20)

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

  • Pressed and released are the wrong way around. This project
    Check that NC goes to ground and NO goes to 3.3 V. Swapping them flips the reading.
  • Nothing is printed. This project
    Make sure the common leg (COM) is on GP14, along with the resistor and the capacitor.
  • The input reads the wrong level, or floats when released. Limit switch
    Use COM and one contact. NC is closed at rest, NO closes when pressed. Add a pull-up or INPUT_PULLUP so the pin never floats.
  • Readings chatter when the lever moves. Limit switch
    Contacts bounce for a few milliseconds. Debounce in software with a 10 to 20 ms delay, or add a small capacitor across the switch.
  • The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
    Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
  • A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
    Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
  • An electrolytic capacitor heats up, bulges or pops. Capacitor
    Electrolytics have a polarity. The stripe marks the negative leg, which goes toward ground. Also keep the voltage under the rated value, such as 10 V or 16 V.
  • An RC circuit charges much slower or faster than expected. Capacitor
    The time constant is R times C. A 10 k resistor with a 100 uF capacitor gives 1 second, and about 5 seconds to charge fully. Check that you read the value on the part right.

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